Light and electron microscopic studies of structural alterationa occurring in 30 porcine valvular heterografts after various periods of implantation in patients showed that alterations in valvular collagen are of crucial importance in determining the long-term durability of porcine valvular heterografts. The two most important complications in these heterografts are: 1) perforation of the leaflets, which is related to collagen breakdown and leads to valvular regurgitation, and 2) calcification of the leaflets, which involves the collagen and results in prosthetic valvular stenosis.